Until not long ago, just about until electricity became ubiquitous, humans used to have a sleep pattern quite different from what we consider "normal" today. At dusk you go to sleep, at some point in the middle of the night you wake up for an hour or two, then fall asleep again until dawn. Thus there are two events of falling asleep and two events of waking up every night (plus, perhaps, a short nap in the afternoon). As indigenous people today, as well as people in non-electrified rural areas of the world, still follow this pattern, it is likely that our ancestors did, too.

When left to my own devices, i.e., during periods when I've been unemployed and didn't have to get up in the morning to get to work, my natural rhythm follows this pattern. I would get sleepy and go to bed for a "nap" in the early evening, say 7:00, sleep three or four hours, then get up for a few hours, then go back to bed for a second shift. A nap in mid-afternoon wouldn't be unheard of, either.

Everyone doesn't have the same natural sleeping pattern, yet most of the world is forced into the same one for purposes of industry. Which is understandable, of course, but difficult for people who don't fit into the (artificial) norm.

I can adapt pretty easily, it's just not my natural state. I love it when I'm able to do it, but not at the risk of my job, of course.

Link from Boing Boing.